1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image by an electrophotographic process or an electrostatic recording process. More particularly, the present invention is concerned with an image forming apparatus having an image transfer device for transferring a toner image formed on an image carrier from the image carrier to a transfer member which is carried by a transfer member carrying means.
2. Description of the Related Art
FIG. 6 shows a known multi-color electrophotographic copying apparatus. This apparatus has a rotary-type developing device. More specifically, referring to FIG. 6, the multi-color electrophotographic copying apparatus has an image carrier, i.e., a photosensitive drum 3, rotatably supported and capable of rotating in the direction of the arrow, and image forming means arranged around the photosensitive drum 3. The image forming means may be of any type. In the illustrated apparatus, the image forming means includes a primary charger 11 for uniformly charging the surface of the photosensitive drum 3, an exposure device 12 such as, for example, a laser beam exposure unit capable of exposing the photosensitive drum 3 to color-separated light images or an equivalent images so as to form electrostatic latent images on the photosensitive drum 3, and a rotary developing device 13 for developing the latent images on the photosensitive drum to make the images visible.
The rotary developing device 13 includes four developer containers 2Y,2M,2C and 2BK which contains an yellow color developer, a magenta color developer, a cyan color developer and a black color developer, respectively, and developing units 1Y, 1M, 1C and 1Bk which are supplied with the respective developers from the developer containers. These four developing units are arranged for rotation about an axis of a substantially cylindrical casing. These rotary developing device 13 is arranged such that a rotation of the substantially cylindrical casing causes a desired developing unit to be moved to a position where it opposes the outer peripheral surface of the photosensitive drum 3 so as to develop an electrostatic latent image on the photosensitive drum 3. This operation is repeated to produce a full-color (four color) image on the photosensitive drum 3.
The developed visible image (referred to as "toner image" hereinafter) on the photosensitive drum 3 is transferred, at a transfer position, to a transfer member P which is carried by a transfer member carrier sheet 14 (see FIG. 7) which serves as transfer member carrying means. The term "transfer position" is used to mean a position where a transfer drum 4 opposes the photosensitive drum 3 across the transfer member P.
As shown in FIGS. 7 and 8, the transfer drum 4 has ring portions 4a, 4b disposed on both axial ends thereof and a bridge portion 4c interconnecting these ring portions 4a, 4b. A transfer member carrier means 14 is stretched over an open area formed between both ring portions 4a,4b. The transfer member carrier means 14 is typically a film-like dielectric sheet such as of polyethylene terephthalate, polyvinylidene fluoride or the like material. The bridge portion 4c has a transfer member gripper 15 for gripping a transfer member P which is fed from a sheet feeder. The transfer member gripper 15 for mechanically gripping the transfer member P may be substituted by an electrostatic absorption means (not shown) which can electrostatically attract and hold the transfer member P onto the transfer member carrier means 14.
A transfer charger 7 as a transfer means and a charge-removing discharger 10 are disposed inside the transfer drum 4, while charge removing dischargers 9 and 16 are arranged outside the transfer drum 4.
The process for forming a full-color image by the described multi-color electrophotographic copying apparatus will be outlined.
The surface of the photosensitive drum 3 is uniformly charged by a primary charger, and is irradiated with a light image corresponding to a picture information formed by the exposure device 12, whereby an electrostatic latent image is formed on the surface of the photosensitive drum 3. The latent image thus formed is developed by the rotary developing device 13 with a toner containing a resin as a base material and having a mean particle size of 12 .mu.m so that a visible toner image is formed.
Meanwhile, the transfer member P is fed onto the transfer drum 4 by a register roller 17 in synchronization with the formation of the image and is caught at its leading end by the gripper 15 so as to be moved in the direction of the arrow bin accordance with the rotation of the transfer drum 4.
Subsequently, in the transfer position, a corona discharge of the opposite polarity to the toner is effected by the transfer charger 7 from the back side of the transfer member carrying means 14, i.e., the dielectric sheet 14, of the transfer drum 4, whereby the toner image is transferred from the photosensitive drum 3 to the transfer member P.
This operation is repeated a desired number of times and, thereafter, the transfer member P is separated from the transfer drum 4 by the action of a separator claw 18 while electrostatic charges are removed by the charge removing dischargers 9, 10 and 16. The separated transfer member P is then conveyed by a conveyor belt 19 to a fixing device 20 which applies heat to the toner image so as to fix the toner image to the transfer sheet. The transfer sheet is then ejected from the apparatus.
Meanwhile, the portion of the photosensitive drum 3 from which the toner image has been transferred is made to pass through a cleaning device 21 which removes any residual toner from the surface of the photosensitive drum 3, and is then subjected again to the image forming process. At the same time, the dielectric sheet 14 on the transfer drum 4 is cleaned by a cleaning device 22 having, for example, a fur brush, as well as an auxiliary cleaning means 23, so as to be used again in the image forming process.
In order to cope with a current demand for higher grade of image quality, it is desirable to attain a higher degree of fineness of the latent image and to improve the reproducibility of the latent image. To met these demands, it is a current measure to use, as the developer, toners of small particle sizes, e.g., 10 .mu.m or smaller and about 8 .mu.m or smaller in terms of mean size. In general, a smaller particle size increases the quantity of charges per unit mass, as well as Van der Waals force, due to increase in the specific area. As a result, the adhesion of the toner particles to the photosensitive drum is enhanced, with the result that a higher intensity of the transfer electric field or higher degree of closeness of contact between the transfer member and the photosensitive drum is required to obtain good transfer performance. Unfortunately, however, non-uniform or irregular transfer is caused due to local transfer failures caused by local minute gaps formed between the photosensitive drum and the transfer member in the transfer region.
In order to obtain a clear transferred image by improving the transfer efficiency, it has been proposed to use a pressing member, e.g.,an elastic sheet 1, which is disposed upstream of the transfer charger 7 as viewed in the direction of rotation of the transfer drum 4 so as to extend in the downstream direction from the inlet side of the transfer drum 4 and which is capable of pressing the dielectric sheet 14.
The provision of the elastic sheet 1 alone, however, poses the following problem. Namely, the corona discharge by the transfer corona discharger 7 is effected over the entire area of the elastic sheet 1 so that a large quantity of charges is accumulated on the elastic sheet 1 so that a strong electric field is generated to block the corona discharge which is directed from the discharger 7 towards the sheet 14, thus impeding the charging of the dielectric sheet 14 by the corona discharger 7. Furthermore, any slight unevenness of the charge distribution on the elastic sheet 1, attributable to a minor non-uniformity of the discharge, tends to grow large due to accumulation of the large quantity of charges. Such large unevenness of he charge distribution adversely affects the electric field, causing non-uniform charging of the dielectric sheet 14 and, hence, an irregular transfer of the image, resulting in an inferior quality of the transferred image.
A transfer device of the type which transfers and superposes a plurality of toner images as in the case of a multi-color image forming apparatus, particularly when a fine toner of a particle size of 10 .mu.m or smaller is used, requires a higher intensity of the transfer electric field than in the case where a monochromatic image formation is conducted or a toner having greater particle sizes, e.g., 12 .mu.m or greater, is used.
In consequence, the elastic sheet 1 is charged more strongly due to application of the stronger transfer electric field, with the result that the non-uniformity of the charging and, hence, irregularity in the image transfer are enhanced so as to seriously deteriorate the quality of the image.
Japanese Patent Unexamined publication No. 59-119373 disclosed various methods for obviating the above-described problems. For instance, it is proposed to use a conductor adhered to the side of the elastic sheet adjacent the corona charger and connected to a bias power supply. It is also proposed to ground the above-mentioned conductor through a constant-voltage element. In the arrangement in which the conductor is connected to a bias power supply, a bias of the same polarity as the corona charging is applied to the conductor so as to efficiently apply the corona discharge to the transfer member carrier sheet. When a constant-voltage element is used, the conductor is stably held at a constant potential so as to stabilize the effect of the corona discharge.
In these proposed methods, however, the conductor is undesirably influenced by the ozone and nitrogen oxides generated as a result of the corona discharge, because the conductor is adhered to the side of the pressing member which is directly subjected to the corona discharge. As a consequence, the electrical characteristics of the conductor is undesirably changed to impair the effect of provision of the conductor.